1. Field of the Invention
This invention relates to a method for selecting data storage media for demount in an idle automated storage library, and an automated storage library and program product therefor. More particularly, the method is for the selection of data storage media for demount in an automated storage library, regardless of the occupancy status of the peripheral storage devices therein, and an automated storage library and program product therefor.
2. Description of the Related Art
Modern computers require a host processor including one or more central processing units and a memory facility. The processor manipulates data stored in the memory according to instructions provided to it. The memory must therefore be capable of storing data required by the processor and transferring that data to the processor at a rate capable of making the overall operation of the computer feasible. The cost and performance of computer memory is thus critical to the commercial success of a computer system.
Because today's computers require large quantities of data storage capacity, computer memory is available in many forms. A fast but expensive form of memory is main memory, typically comprised of microchips. Other available forms of memory are known as peripheral storage devices and include magnetic direct access storage devices (DASD), magnetic tape storage devices, and optical recording devices. These types of memory actually store data on storage media therein. Each of these other types of memory has a greater storage density and lower cost than main memory. However, these other memory devices do not provide the performance provided by main memory. For example, the time required to properly position the tape or disk beneath the read/write mechanism of the drive cannot compare with the rapid, purely electronic data transfer rate of main memory.
It is inefficient to store all of the data in a computer system on but a single type of memory device. Storing all of the data in main memory is too costly and storing all of the data on one of the peripheral storage devices reduces performance. Thus, a typical computer system includes both main memory and one or more types of peripheral storage devices arranged in a data storage hierarchy. The data storage hierarchy arrangement is tailored to the performance and cost requirements of the user. In such a hierarchy, main memory is often referred to as primary data storage, the next level of the hierarchy is often referred to as secondary data storage, and so on. Generally, the highest level of the hierarchy has the lowest storage density capability, highest performance and highest cost. As one proceeds down through the hierarchy, storage density generally increases, performance generally decreases, and cost generally decreases. By transferring data between different levels of the hierarchy as required, the cost of memory is minimized and performance is maximized. Data is thus stored in main memory only so long as it is expected to be required by the processor. The hierarchy may take many forms, include any number of data storage or memory levels, and may be able to transfer data directly between any two distinct memory levels. The transfer of data may employ I/O channels, controllers, or cache memories as is well known in the art.
Images may be included in engineering drawings, financial and insurance documents, medical charts and records, etc. Until recently, it was not possible to store image data in memory in a cost effective manner. Images can take many forms, and therefore cannot be encoded into the binary 0's and 1's of computers as easily and compactly as text. Engineering drawings are typically stored on paper, microfilm, or microfiche requiring manual retrieval when access to a drawing is necessary. The same is true for X-rays and other diagnostic medical images, bank checks used in transactions between financial institutions, insurance records, images in FAX documents and so on. Thus, despite modern computers, it is estimated that most of the world's data is still stored on paper. The cost of filing, storing, and retrieving such paper documents including image data is escalating rapidly. It is no longer acceptable to maintain rooms or warehouses stocked full of documents which must be retrieved manually when access thereto is required. Optical scanners are now capable of converting images into machine readable form for storage on peripheral storage devices, but the storage space required for the image data--although significantly less than that required for paper documents--is still quite large. Numerous disks or tapes are required for most business applications. Automated storage libraries have thus been developed to manage the storage of such disks or tapes.
Automated storage libraries include a plurality of storage cells or slots for retaining data storage media, such as magnetic tapes, magnetic disks, or optical disks, a robotic picker mechanism, and one or more internal peripheral storage devices. Each data storage medium may be contained in a cassette or cartridge housing for easier handling by the picker. The picker operates on command to transfer the data storage media between the storage cells and the internal peripheral storage devices without manual assistance. An internal peripheral storage device having a storage medium mounted therein is referred to as "occupied". Once a data storage medium is mounted in an internal peripheral storage device, data may be written to or read out from that medium for as long as the system so requires. Data is stored on a medium in the form of one or more files, each file being a logical data set. A file is considered "open" when it is reserved for access by a particular user and the storage medium upon which it resides is mounted in a peripheral storage device and ready to be accessed. For example, in an optical disk library, a file is open if it is reserved for exclusive access and the disk on which it resides is mounted in a drive and spinning. A peripheral storage device having a storage medium therein with an open file is referred to as "active", regardless of whether actual electonric transfer is occurring. A peripheral storage device is also active if the storage medium mounted therein is undergoing access under any standard operating system command not requiring that a file be open, such as a directory read. An active storage medium is generally considered to be one in an active peripheral storage device. The internal peripheral storage devices and storage cells may be considered distinct levels of a data storage hierarchy. In addition, data storage media in shelf storage (i.e. not in the storage cells, but instead outside the reach of the robotic picker without manual intervention) may be considered yet another level of a data storage hierarchy.
Automated storage libraries may also include one or more external peripheral storage devices. An external peripheral storage device is a peripheral storage device which, unlike internal peripheral storage devices, is not accessible by the picker but must instead be loaded and unloaded manually. External peripheral storage devices may be included in libraries as a convenience to the library operator. A shelf storage medium requiring brief access will not have to be inserted into the library and retrieved by the picker for mounting in one of the internal peripheral storage devices. External peripheral storage devices may also be considered a distinct level of a data storage hierarchy. Except as explicitly mentioned herein, "peripheral storage devices" hereinafter refers to internal peripheral storage devices only.
Several automated storage libraries are known. IBM Corporation introduced the 3850 Mass Storage Subsystem for the storage and retrieval of magnetic tape modules in the 1970's. More recently, several firms have introduced automated storage libraries for magnetic tape cartridges and optical disks. For example, magnetic tape cartridge libraries are disclosed in U.S. Pat. Nos. 4,654,727 and 4,864,438, and 4,864,511. Examples of optical disk libraries can be found in U.S. Pat. Nos. 4,271,489, 4,527,262, 4,614,474, and 4,766,581. The robotic picker mechanisms of these libraries include one or more grippers, each gripper capable of handling one data storage medium at a time. The '489, '262, '474 patents disclose robotic pickers having but a single gripper and the '727, '438, 511 and '581 patents disclose robotic pickers having multiple grippers. IBM also markets the 9246 Optical Library Unit which is a two gripper library.
The operation of pickers and grippers greatly effects the efficiency of an automated storage library. The selection of storage media for transfer between storage cells and peripheral storage devices, the timing of the transfers, and the selection of the location to be transferred to are among the factors which determine data access time. Normally, in single gripper libraries, the mounting of a new storage medium in an already occupied peripheral storage device requires that the already mounted medium be demounted and transferred to a storage cell and that the new medium then be retrieved from its storage cell and then mounted. In multiple gripper libraries, however, it is known to have the picker first retrieve a storage medium to be mounted with a first gripper before demounting a storage medium from a peripheral storage device with a second gripper. The storage medium in the peripheral storage device is demounted by the second gripper and immediately replaced by the storage medium in the first gripper (i.e. prior to transferring the demounted storage medium to a storage cell). By retrieving the next storage medium to be mounted in the peripheral storage device prior to emptying that device, the period of time for which the peripheral storage device is left unmounted (i.e. without any storage medium therein) is reduced. The savings can be several seconds, a significant amount in the data processing environment.
Also, in multiple gripper disk libraries, the disk to be demounted can be spun down while the disk to be mounted is being retrieved. Magnetic and optical disks normally spin at extremely high velocities while mounted to allow the read/write mechanism to rapidly locate data. The spinning of the disks must be discontinued prior to demount to avoid damaging the disks as they are removed from the drives. Stopping disks from spinning at these high velocities requires several seconds. Such "spinning down" of a disk to be demounted while another disk is being retrieved is not possible in a library having but a single gripper. Thus, while single gripper libraries may cost less than multiple gripper libraries to manufacture because of the elimination of additional gripper hardware and programming therefor, the performance of such single gripper libraries is reduced as well.
Other factors are known to effect library efficiency. It is possible to create a library wherein a mounted storage medium is simply demounted and transferred to a storage cell as soon as access to it is no longer currently required (i.e. when the peripheral storage device becomes inactive). There is always a possibility, however, that access to a storage medium will be required shortly after it is demounted. To reduce the likelihood of having to remount a medium shortly after it is demounted, known as "churn" or "thrashing", it is known to leave all mounted storage media in their respective peripheral storage devices until two conditions exist. These two conditions are the occupation of all peripheral storage devices in the library and the issuance of a mount request for a currently unmounted storage medium. The use of such conditions recognizes that there is no need to demount any storage medium so long as unoccupied peripheral storage devices are not required to accept newly requested mounts. In addition, the selection of the storage medium to be demounted to allow for a new mount is known to be made using least recently used (LRU) or least recently mounted criteria. Such selection is made to retain in mounted condition those storage media to which continued access is considered most likely. Examples of these operations can be found in commonly assigned, copending U.S. patent application Ser. No. 190,739, filed May 5, 1988.
The described conditional demount operations may at times improve performance over that of simply demounting the storage media as soon as the peripheral storage devices in which they are mounted become inactive, but nevertheless also incur performance tradeoffs which have not been addressed by the aforementioned references and products. The tradeoffs occur because the described conditional demount operations will never allow a storage medium to be demounted until the mounting of another medium is required. Some tradeoffs are related to the aforementioned significant performance disadvantage of single gripper libraries as compared to multiple gripper libraries. Delaying demount of any mounted data storage media according to the described conditions means that the aforementioned performance disadvantage cannot be avoided--once a mount request is issued, the mounted storage medium must be demounted and transferred to a storage cell before the new medium can be retrieved and mounted. This is particularly wasteful in single gripper libraries containing data storage media which are accessed randomly and thus not likely to suffer from churn. Also, mounted disks cannot be spun down during retrieval of a new disk to be mounted. The failure of any library to maintain an unoccupied peripheral storage device means that the storage device in one of such devices must at least be demounted prior to mounting a new storage medium, thereby slowing the servicing of mount requests. Even if a library has multiple grippers and can therefore delay in transferring a demounted storage medium to a storage cell, mounting of a new storage medium is delayed until the storage medium already mounted in the peripheral storage device is demounted.
Another significant tradeoff of the described conditional demount operations is that of poor library reliability. Failing to allow a storage medium to be demounted until the mounting of another medium is required will unduly burden certain peripheral storage device resources. In disk libraries, the still mounted disks continue to be spun by the disk drive motor and tracked by the disk drive servo system. In tape cartridge libraries, the still mounted tapes continue to be supported by a hydrostatic air bearing. In any libraries, certain drive electronics may also be maintained in a powered, ready state. The additional use of these drive resources results in premature aging of such resources, thereby reducing reliability. Disk drive motors, optical disk drive lasers, and tape drive air compressors will wear out at increased rates.